This invention relates to a system for reporting alarm information from a plurality of remote stations to a master station and, more particularly, to a system wherein each remote station has a clock synchronized by the clock of the master station for providing a plurality of time slots after a synchronization signal is received and for determining which remote station should respond during which time slot and, also more particularly, to a system where the information reported by the remote stations is coded at the remote stations and decoded by the master station using code generators which are arranged in substantially the same manner at both the master station and the remote stations.
Data processing equipment, especially that which is designed for use in monitoring and controlling building air conditioning equipment and fire and security points within a building, is typically designed for monitoring a large number of input points. And when the sytsem is to be used to monitor such points spread throughout the building, it is too expensive for such systems to rely upon multiconductor cables used in typical computer systems for interconnecting the computer with its peripheral equipment. Typical in building control and monitoring systems, the central station or computer is connected over a coax cable to its plurality of remote stations. Thus, there is no dedicated wiring to each remote station which can be used to construct the time sequence in which the remote stations report their information to the central station and thereby avoid the situation where two or more remote stations attempt to communicate with the central station at the same
To avoid the simultaneous transmission of information by two or more remote stations, the prior art has relied upon various techniques. In one technique, the central station polls the remote stations in sequence and requests the reporting of information. The central station will not poll the next station until it has received the information from the previous station. Since a remote station cannot respond or report information to the central station until it has been polled, there is little danger that multiple remote stations will attempt to transmit at the same time. However, the time required for the central station to send out as many poll messages as there are remote stations can be prohibitive. Therefore, systems have been devised for allowing the central station to transmit one global polling message which then causes the remote stations to transmit their information in sequence on a priority basis; that is, the second station will not report its information until the first station has finished reporting.
The drawback to this type of system is that if prior stations have large amounts of information to transmit to the central station, subsequent stations have to wait long periods of time for transmitting what little bit of information they may have. To avoid this problem, time multiplexing systems provide time slots having a fixed duration so that prior remote stations will not unnecessarily delay the transmission of information by subsequent stations. Thus, if prior stations have more information to transmit than can be transmitted within their time slot, they must wait for their subsequent time slots in which to transmit the rest of their information. However, all of these systems are more suited to large scale application. The present system is useful in those buildings where only a limited number of points need to be monitored.